Recently, there has been increased demand for thin and lightweight display devices achieving high-speed response. Therefore, there has been actively conducted research and development on organic EL (Electro Luminescence) displays and FEDs (Field Emission Displays).
An organic EL element included in an organic EL display emits light at higher luminance as a voltage to be applied thereto is high and an electric current flowing therethrough is large in amount. In the organic EL element, however, a relation between the luminance and the voltage varies readily due to influences such as a driving time and an ambient temperature. Consequently, it becomes very difficult to suppress the variations in luminance of the organic EL element if a driving scheme of a voltage control type is adopted for the organic EL display. In contrast to this, the luminance of the organic EL element is almost proportional to the electric current. This proportional relation is less susceptible to an influence of an extraneous factor such as an ambient temperature. Accordingly, it is preferable that a driving scheme of an electric current control type is adopted for the organic EL display.
Meanwhile, a display device includes a pixel circuit and a drive circuit each of which is configured using a TFT (Thin Film Transistor) made of amorphous silicon, low-temperature polycrystalline silicon, CG (Continuous Grain) silicon or the like. However, such a TFT has characteristics (e.g., threshold voltage, mobility) which vary readily. For this reason, a circuit that compensates the variations in characteristic of the TFT is provided for the pixel circuit of the organic EL display. Thus, the variations in luminance of the organic EL element are suppressed by action of this circuit.
In the driving scheme of the electric current drive type, a scheme for compensating variations in characteristic of a TFT is broadly divided into an electric current program scheme in which an amount of an electric current flowing through a driving TFT is controlled by an electric current signal and a voltage program scheme in which the amount of this electric current is controlled by a voltage signal. Use of the electric current program scheme allows compensation of variations in threshold voltage and mobility. Use of the voltage program scheme allows compensation of only the variations in threshold voltage.
However, the electric current program scheme has the following two problems. First, it is difficult to design a pixel circuit and a drive circuit since an electric current to be used herein is considerably small in amount. Second, it is difficult to make a large-area circuit since an influence of a parasitic capacity is exerted readily when an electric current signal is set. In contrast to this, according to the voltage program scheme, an influence of a parasitic capacity or the like is minute and a circuit is designed in a relatively ease manner. Moreover, an influence of variations in mobility to be exerted on an amount of an electric current is smaller than an influence of variations in threshold voltage to be exerted on the amount of the electric current. Further, the variations in mobility can be suppressed to a certain degree in a step of manufacturing a TFT. Accordingly, even a display device that adopts the voltage program scheme can provide satisfactory display quality.
With regard to an organic EL display that adopts the driving scheme of the electric current drive type, conventionally, there has been known the following pixel circuit. FIG. 11 is a circuit diagram showing a pixel circuit described in Patent Document 1. The pixel circuit 90 shown in FIG. 11 includes a driving TFT 91, switching TFTs 92 to 94, capacitors 95 and 96, and an organic EL element 97 (also referred to as an OLED (Organic Light Emitting Diode)). Each of the TFTs included in the pixel circuit 90 is of a P-channel type.
In the pixel circuit 90, the driving TFT 91, the switching TFT 94 and the organic EL element 97 are provided in series between a power supply wiring line Vp (potential: VDD) and a common cathode (GND). The capacitor 95 and the switching TFT 92 are provided in series between a gate terminal of the driving TFT 91 and a data line Sj. The switching TFT 93 is provided between the gate terminal and a drain terminal of the driving TFT 91, and the capacitor 96 is provided between the gate terminal of the driving TFT 91 and the power supply wiring line Vp. The switching TFT 92 has a gate terminal connected to a scanning line Gi, the switching TFT 93 has a gate terminal connected to an auto-zero line AZi and the switching TFT 94 has a gate terminal connected to an illumination line ILi.
FIG. 12 is a timing chart showing a timing that data is written to the pixel circuit 90. Prior to a time t0, a potential at the scanning line Gi and a potential at the auto-zero line AZi are controlled to a high level, respectively, a potential at the illumination line ILi is controlled to a low level, and a potential at the data line Sj is controlled to a reference potential Vstd. At the time t0, when the potential at the scanning line Gi is changed to the low level, the switching TFT 92 is changed to a conduction state. At a time t1, next, when the potential at the auto-zero line AZi is changed to the low level, the switching TFT 93 is changed to the conduction state. In the driving TFT 91, thus, the gate terminal and the drain terminal become equal in potential to each other.
At a time t2, next, when the potential at the illumination line ILi is changed to the high level, the switching TFT 94 is changed to a non-conduction state. Herein, an electric current flows from the power supply wiring line Vp into the gate terminal of the driving TFT 91 via the driving TFT 91 and the switching TFT 93. The potential at the gate terminal of the driving TFT 91 rises during a period that the driving TFT 91 is in the conduction state. The driving TFT 91 is changed to the non-conduction state when a gate-source voltage becomes a threshold voltage Vth (negative value) (i.e., when the potential at the gate terminal becomes (VDD+Vth)). Accordingly, the potential at the gate terminal of the driving TFT 91 rises to (VDD+Vth).
At a time t3, next, when the potential at the auto-zero line AZi is changed to the high level, the switching TFT 93 is changed to the non-conduction state. Herein, a difference in potential (VDD+Vth−Vstd) between the gate terminal of the driving TFT 91 and the data line Sj is held at the capacitor 95.
At a time t4, next, when the potential at the data line Sj is changed from the reference potential Vstd to a data potential Vdata, the potential at the gate terminal of the driving TFT 91 is changed by the same amount (Vdata−Vstd) and then becomes (VDD+Vth+Vdata−Vstd). At a time t5, next, when the potential at the scanning line Gi is changed to the high level, the switching TFT 92 is changed to the non-conduction state. Herein, the gate-source voltage (Vth+Vdata−Vstd) of the driving TFT 91 is held at the capacitor 96. At a time t6, next, the potential at the data line Sj is changed from the data potential Vdata to the reference potential Vstd.
At a time t7, next, when the potential at the illumination line ILi is changed to the low level, the switching TFT 94 is changed to the conduction state. Thus, an electric current flows from the power supply wiring line Vp into the organic EL element 97 via the driving TFT 91 and the switching TFT 94. An amount of the electric current flowing through the driving TFT 91 increases/decreases in accordance with the potential (VDD+Vth+Vdata−Vstd) at the gate terminal. However, the amount of the electric current is the same as long as the potential difference (Vdata−Vstd) is the same even when the threshold voltage Vth is different. Irrespective of the value of the threshold voltage Vth, accordingly, the electric current flows through the organic EL element 97 in an amount which depends on the potential Vdata, so that the organic EL element 97 emits light at a luminance which depends on the data potential Vdata.
In addition to this, with regard to the organic EL display, there have been known a method for providing a threshold value correction circuit outside a pixel circuit, and a method for setting a threshold value correction period longer than a period for selecting a pixel circuit. For example, Patent Document 2 describes the following method. That is, an electric current capability of a drive element is measured and is stored in a memory provided outside a pixel circuit, and a voltage to be supplied to a panel is changed in accordance with the stored electric current capability (see FIG. 13). Moreover, Patent Document 3 describes the following method. That is, a switch for applying an initial voltage to one end of a coupling capacitance is provided for setting a threshold value correction period longer than a selection period.    [Patent Document 1] International Publication No. 98/48403 Pamphlet    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2002-278513    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2004-133240